Process for the production of hydrocarbon blends with a high octane number by the hydrogenation of hydrocarbon blends containing branched olefinic cuts

ABSTRACT

Process for the production of hydrocarbon blends with a high octane number by the hydrogenation of hydrocarbon blends, containing branched C 8 , C 12  and C 16  olefinic cuts, characterized by sending said blends, as such or fractionated into two streams, one substantially containing the branched C 8  olefinic cut, the other substantially containing the branched C 12  and C 16  olefinic cuts, to a single hydrogenation zone or to two hydrogenation zones in parallel, respectively, 
     only the stream substantially containing of saturated C 8  hydrocarbons, obtained by the fractionation of the stream produced by the single hydrogenation zone or obtained by the hydrogenation zone fed by the fractionated stream substantially containing the branched C 8  olefinic cut, being at least partly recycled to the single hydrogenation zone or to the hydrogenation zone fed by the fractionated stream substantially containing the branched C 8  olefinic cut, and the hydrocarbon blend with a high octane number, obtained by the fractionation of the stream produced from the single hydrogenation zone or obtained from the hydrogenation zone, being fed by the fractionated stream substantially containing the branched C 12  and C 16  olefinic cuts.

The present invention relates to a process for the production ofhydrocarbon blends with a high octane number by the hydrogenation ofhydrocarbon blends containing branched C₈, C₁₂ and C₁₆ olefinic cuts,optionally obtained by the selective dimerization of hydrocarbon cutscontaining isobutene.

Refineries throughout the world are currently in the process ofproducing “Low Environmental Impact Fuels” (characterized by a reducedcontent of aromatics, olefins, sulfur and a lower volatility), obviouslyattempting to minimize the effect of their production on the functioningof the refinery itself.

MTBE and alkylated products are the most suitable compounds forsatisfying the future demands of refineries, however the use of MTBE isat present hindered by unfavourable legislative regulations whereasalkylated products have a limited availability.

As a result of the continuous attacks on MTBE, due to its poorbiodegradability and presumed toxicity, this compound has been bannedfrom fuels in California and in many other states in the USA (50%approximately of the world market); consequently not only is itdifficult to foresee its use (together with that of other alkyl ethers)in reformulated fuels in the near future, but rather, the removal ofthis ether will create considerable problems for refineries as, inaddition to its high octane function, MTBE also exerts a diluting actionof the most harmful products for the environment (sulfur, aromatics,benzene, etc.).

Alkylated products are undoubtedly ideal compounds for reformulatedfuels as they satisfy all the requisites envisaged by futureenvironmental regulations as they combine a high octane number with alow volatility and the practically complete absence of olefins,aromatics and sulfur.

A further positive aspect of alkylation is that it is capable ofactivating isoparaffinic hydrocarbons, such as, for example, isobutanewhich binds itself, by reaction in liquid phase catalyzed by strongacids, with olefins (propylene, butanes, pentanes and relative blends)producing saturated C₇-C₉ hydrocarbons with a high octane number.

Higher productions of alkylated products than those currently available,however, would require the construction of large alkylation units as,due to its scarcity, an alkylated product does not represent a commoditywhich is widely available at present on the market, but forms acomponent of gasoline used for captive use in the refineries whichproduce it.

This represents a great limitation for the large-scale use of alkylatedproducts as the construction of new units is limited by theincompatibility of the catalysts used in traditional processes(hydrochloric acid and sulfuric acid) with the new environmentalregulations: processes with hydrochloric due to the dangerous nature ofthis acid, especially in populated areas, processes with sulfuric acidas a result of its highly corrosive capacity as well as the considerableproduction of acid mud which is difficult to dispose of.

Alternative processes with solid acid catalysts are being developed buttheir commercial applicability must still be demonstrated.

In order to face this problem, increasing resort will have to be made topurely hydrocarbon products, such as those obtained by the selectivedimerization of C₃ and C₄ olefins, which, as a result of their octanecharacteristics (both a high Research Octane Number (RON) and also MotorOctane Number (MON)) and their boiling point (poor volatility but lowend point) are included in the range of compositions which are extremelyinteresting for obtaining gasolines which are more compatible withcurrent environmental demands.

Oligomerization (often incorrectly called polymerization) processes werewidely used in refining in the thirties' and forties' to convertlow-boiling C₃-C₄ olefins into so-called “polymer” gasoline. Typicalolefins which are oligomerized are mainly propylene, which gives (C₆)dimers or slightly higher oligomers depending on the process used, andisobutene which mainly gives (C₈) dimers but always accompanied byconsiderable quantities of higher oligomers (C₁₂ ⁺).

This process leads to the production of a gasoline with a high octanenumber (RON about 97) but also with a high sensitivity due to the purelyolefinic characteristic of the product (for more specified details onthe process see: J. H. Gary, G. E. Handwerk, “Petroleum Refining:Technology and Economics”, 3^(rd) Ed., M. Dekker, New York, (1994),250). The olefinic nature of the product represents an evident limit tothe process as the hydrogenation of these blends always causes aconsiderable reduction in the octane characteristics of the product,which thus loses its activity.

If we limit our attention to the oligomerization of isobutene, it isknown that this reaction is generally carried out with acid catalystssuch as phosphoric acid supported on a solid (for example kieselguhr),cationic exchange acid resins, liquid acids such as H₂SO₄ or sulfonicacid derivatives, silico-aluminas, mixed oxides, zeolites, fluorinatedor chlorinated aluminas, etc.

The main problem of dimerization, which has hindered its industrialdevelopment, is the difficulty in controlling the reaction rate; thehigh activity of all these catalytic species together with thedifficulty in controlling the temperature in the reactor, does in factmake it extremely difficult to limit the addition reactions of isobuteneto the growing chains and consequently to obtain a high-quality productcharacterized by a high selectivity to dimers.

In dimerization reactions, there is in fact the formation of excessivepercentages of heavy oligomers such as trimers (selectivity of 15-60%)and tetramers (selectivity of 2-10%) of isobutene. Tetramers arecompletely outside the gasoline fraction as they are too high-boilingand therefore represent a net loss in yield to gasoline; as far astrimers (or their hydrogenated derivatives) are concerned, it isadvisable to strongly reduce their concentration as they arecharacterized by a boiling point (170-180° C.) at the limit of futurespecifications on the final boiling point of reformulated gasolines.

In order to obtain a better-quality product by reaching higherselectivities (content of dimers >80-85% by weight), it is possible touse different solutions which can moderate the activity of the catalystand consequently control the reaction rate:

-   -   oxygenated compounds can be used (tertiary alcohol and/or alkyl        ether and/or primary alcohol) in a sub-stoichiometric quantity        with respect to the isobutene fed in the charge using tubular        and/or adiabatic reactors (IT-MI95/A001140 of 1 Jun. 1995,        IT-MI97/A001129 of 15 May 1997 and IT-MI99/A001765 of 5 Sep.        1999);    -   tertiary alcohols can be used (such as terbutyl alcohol) in a        sub-stoichiometric quantity with respect to the isobutene fed in        the charge using tubular and/or adiabatic reactors        (IT-MI94/A001089 of 27 May 1994;    -   alternatively, it is possible to suitably modify the charge by        mixing the fresh charge with at least a part of the hydrocarbon        stream obtained after the separation of the product, so as to        optimize the isobutene content (<20% by weight) and use a linear        olefin/isobutene ratio greater than 3: in this case, the use of        reactors such as tubular or “Boiling Point Reactors” capable of        controlling the temperature increase, is fundamental for        obtaining high selectivities (IT-MI2000/A001166 of 26 May 2000).

Using these solutions, it is therefore possible to favour thedimerization of isobutene and isobutene/n-butene co-dimerizations, withrespect to the oligomerization, and avoid the triggering ofoligomerization-polymerization reactions of linear butenes which arefavoured by high temperatures.

The dimerization product is then preferably hydrogenated to give acompletely saturated final product, with a high octane number and lowsensitivity. For illustrative purposes, the octane numbers and relativeboiling points of some of the products obtained by the dimerization ofisobutene are indicated in the following table.

PRODUCT RON MON b.p. (° C.) Diisobutylenes 100 89 100-105 Iso-octane 100100 99 Tri-isobutylenes 100 89 175-185 Hydrogenated 101 102 170-180tri-isobutylenesThe hydrogenation of olefins is generally effected using two groups ofcatalysts:

-   -   those based on nickel (20-80% by weight);    -   those based on noble metals (Pt and/or Pd) supported on a metal        content of 0.1-1% by weight.

The operating conditions used for both groups are quite similar; in thecase of nickel catalysts, resort must be made however to a higherhydrogen/olefin ratio as these catalysts have a greater tendency towardsfavouring the cracking of the olefins. Nickel-based catalysts are lesscostly but become more easily poisoned in the presence of sulfuratedcompounds; the maximum quantity of sulfur they can tolerate is 1 ppmwith respect to approximately 10 ppm tolerated by catalysts based onnoble metals. The selection of the type of catalyst to be used thereforedepends on the particular charge to be hydrogenated.

A wide range of operating conditions can be adopted for thehydrogenation of olefins; it is possible to operate in vapour phase orin liquid phase but operating conditions in liquid phase are preferred.The reactor configuration can be selected from adiabatic fixed bedreactors, tubular reactors, stirred reactors or column reactors, even ifthe preferred configuration envisages the use of an adiabatic reactorwhich can optionally consist of one or more catalytic beds (separated byintermediate cooling).

The hydrogen pressure is preferably below 5 MPa, more preferably between1 and 3 MPa. The reaction temperature preferably ranges from 30 to 200°C. The feeding space velocities of the olefinic streams are preferablylower than 20 h⁻¹, more preferably between 0.2 and 5 h⁻¹. The heat whichdevelops from the reaction is generally controlled by diluting theolefinic charge by recycling a part of the hydrogenated product itself(in a ratio: volume of saturated product/volume of olefin lower than15).

The content of residual olefins in the product depends on the use of theproduct itself; in the case of blends deriving from the dimerization ofisobutene (which can be used as components for gasolines) and having thefollowing average composition

C₈ 80-95% by weight C₁₂ 5-20% by weight C₁₆ 0.1-2% by weighta content of residual olefins lower than 1% can be considered as beingacceptable.

The hydrogenation of a cut having this composition is not a simpleoperation however, as a series of factors should be taken into account:

-   -   the hydrogenation rate is inversely proportional to the chain        length; the hydrogenation of C₈ olefinic dimers does in fact        require much lower temperatures (100-140° C.) with respect to        those necessary for the hydrogenation of C₁₂ olefins (100-200°        C.). In the case of C₁₆ olefins, even higher temperatures are        obviously necessary. Within the single fractions, moreover,        olefins with a terminal double bond are the easiest to        hydrogenate.    -   The reaction temperature must consequently be selected so as to        maximize the conversion of C₁₂ and C₁₆ olefins; in any case it        is onerous to operate under such conditions as to completely        eliminate these olefins.    -   The hydrogenation reaction is extremely exothermic and        consequently to limit the temperature increase in the adiabatic        reactor, the olefinic charge is generally diluted with the        hydrogenated product.    -   The most common hydrogenation catalysts (based on nickel or        palladium) tend to become deactivated as a result of the heavy        olefins and various poisons such as sulfurated compounds. The        greater the number of carbon atoms of the olefins, the slower        the hydrogenation kinetics and the greater possibility there is        of these olefins being deposited on the catalyst forming coke        and reducing its activity. As far as sulfurated compounds are        concerned, on the other hand, the presence of sulfur is        practically inevitable in this type of charge (almost always        greater than 1 ppm and higher in charges from FCC and coking),        nickel catalysts are consequently difficult to use whereas those        based on supported noble metals are preferred. In the case of        charges particularly rich in sulfurated compounds, resort can        also be made to bimetallic catalysts such as those used in        hydrotreating reactions, for example Ni/Co and/or Ni/Mo.

An effective temperature control is consequently the fundamental pointof this type of process. The temperature in the reactor must in fact bekept sufficiently high to kinetically sustain the hydrogenation of heavyolefins but at the same time an excessive increase must be avoided (dueto the exothermicity of the reaction) which can activate possiblecracking phenomena of the olefins or degeneration of the catalyst(sintering of the metal).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. A classical hydrogenation scheme;

FIG. 2. A simplified process scheme;

FIG. 3. A simplified process scheme with a new configuration.

The temperature control in the reactor is generally effected by dilutingthe olefinic charge with the hydrogenated product (in ratios generallyranging from 0.5 to 20) and FIG. 1 indicates a classical hydrogenationscheme.

The stream (1) containing isobutene, for example coming fromSteam-Cracking or Coking or FCC units or from the Dehydrogenation ofisobutane, is sent to the reactor (R1) in which the isobutene isselectively converted to dimers.

The effluent (2) from the reactor is sent to a separation column (C1)where a stream (3) essentially containing the non-converted isobutene,linear olefins and saturated C₄ products (n-butane and isobutane) isremoved at the head, whereas an olefinic stream (4) consisting of dimersand higher oligomers is removed from the bottom, and is fed to thehydrogenation reactor (R2) together with the saturated product (5) andhydrogen (6). The effluent from the reactor (7) is sent to a stabilizingcolumn (C2) from which non-converted hydrogen (8) is recovered at thehead whereas the hydrogenated product (9) is obtained at the bottom. Apart of this stream (10) leaves the plant whereas the remaining streamis recycled to the reactor.

This plant configuration is valid in the case of the hydrogenation of asingle olefinic species (conversions higher than 99%) but may not beeffective when, as in the case of the dimerization product of isobutene,there are olefins with hydrocarbon chains and very different reactionrates. In this case, in fact, the difficulty in completely convertingthe C₁₂ and C₁₆ olefins negatively influences the feasibility of thewhole process; if, in fact, the hydrogenation of C₁₂ and C₁₆ olefins isnot complete, they are recycled to the reactor with a doubly negativeeffect:

-   -   the tendency of accumulating in the product until it is sent        outside specification (total olefins >1% by weight);    -   a reduction in the life of the catalyst as these olefins are        those which have the greatest tendency to become deposited on        the catalyst creating carbonaceous deposits and thus reducing        the activity.

An analogous situation can also be caused by the presence of possiblepoisons (such as sulfurated compounds) which are not completelyconverted in the hydrogenation reactor.

We have now found a process which is economically more advantageous withrespect to a conventional hydrogenation, which envisages the recyclingof the whole C₈-C₁₆ fraction to the reactor, as it is possible to useless drastic reaction conditions and prolong the life of the catalyst.

The process, object of the present invention, for the production ofhydrocarbon blends with a high octane number by the hydrogenation ofhydrocarbon blends, containing branched C₈, C₁₂ and C₁₆ olefinic cuts,is characterized by sending said blends, as such or fractionated intotwo streams, one substantially containing the branched C₈ olefinic cut,the other substantially containing the branched C₁₂ and C₁₆ olefiniccuts, to a single hydrogenation zone or to two hydrogenation zones inparallel, respectively, only the stream substantially containingsaturated C₈ hydrocarbons, obtained by the fractionation of the streamproduced by the single hydrogenation zone or obtained by thehydrogenation zone fed by the fractionated stream substantiallycontaining the branched C₈ olefinic cut, being at least partly recycledto the single hydrogenation zone or to the hydrogenation zone fed by thefractionated stream substantially containing the branched C₈ olefiniccut, and the hydrocarbon blend with a high octane number, obtained bythe fractionation of the stream produced from the single hydrogenationzone or obtained from the hydrogenation zone, being fed by thefractionated stream substantially containing the branched C₁₂ and C₁₆olefinic cuts.

The C₈, C₁₂ and C₁₆ olefinic cuts contained in the hydrocarbon blends tobe treated are preferably oligomers of isobutene, which can derive fromthe dimerization of isobutene.

In addition to said olefinic cuts, the hydrocarbon blends to be treatedcan also contain C₉-C₁₁, and branched C₁₃-C₁₅ olefinic cuts in lowerquantities.

In particular, blends substantially consisting of branched C₈-C₁₆olefins are preferably processed according to the invention, whereinbranched C₁₂ olefins range from 3 to 20% by weight, branched C₁₆ olefinsrange from 0.5 to 5% by weight, the remaining percentage being branchedC₈ olefins.

When two hydrogenation zones in parallel are adopted, it is advisablefor part of the stream substantially containing saturated C₈hydrocarbons, obtained from the hydrogenation zone fed by thefractionated stream substantially containing the branched C₈ olefiniccut, to be sent to the hydrogenation zone fed by the fractionated streamsubstantially containing the branched C₁₂ and C₁₆ olefinic cuts.

The present invention can be effected by fractionating the high-octaneblend either when it is in olefinic form or in hydrogenated form and inboth cases its application makes the hydrogenation step of C₈-C₁₆olefinic streams technically much simpler.

It is in fact possible to use much blander reaction conditions as thereis no longer the necessity of having to maximize the conversion,furthermore the life of the catalyst can be prolonged due to the factthat the heavy hydrocarbons and possible residual olefins are notrecycled to the reactor.

More specifically, the process according to the invention in the case offractionation of the blend in olefinic form, can comprise the followingsteps:

-   -   a) dimerizing the isobutene contained in a C₄ cut (FCC, Coking,        Steam-Cracking, Dehydrogenation of isobutane);    -   b) sending the product leaving the dimerization reactor to a        first distillation column from whose head the C₄ products are        recovered, together with, as side cut, a stream rich in branched        C₈ olefins and as bottom product a stream rich in branched C₁₂        and C₁₆ olefins;    -   c) hydrogenating, in a first reactor, the stream rich in        branched C₈ olefins, obtained as side cut, with suitable        catalysts using a part of the C₈ products themselves already        saturated to dilute the olefinic charge;    -   d) hydrogenating with suitable catalysts, in a second reactor,        the stream rich in branched C₁₂ and C₁₆ olefins together with        the remaining part of the already saturated C₈ products,        obtaining a saturated high-octane hydrocarbon blend.

If the quantity of C₈ products sent to the second reactor is kept equalto that of those removed as side cut of the column, it is possible tohave a hydrogenated product having the same distribution as thehydrocarbons (selectivity to C₈) of the olefinic product leaving thedimerization step.

The stream rich in branched C₈ olefins removed as side cut can besubstantially free of hydrocarbon compounds higher than C₈.

A simplified process scheme is shown in FIG. 2 to illustrate this casemore clearly.

The C₄ stream (1) containing isobutene is sent to the reactor (R1) inwhich the isobutene is selectively converted to dimers. The effluent (2)from the reactor is sent to a separation column (C1) where a stream (3)essentially containing the non-converted isobutene, linear olefins andsaturated C₄ products (n-butane and isobutane) is removed at the head,C₈ olefins (4) are recovered as side cut whereas a stream (5) in whichthe higher oligomers (C₁₂ and C₁₆) are concentrated, is removed at thebottom.

The side cut (4) is sent to the first hydrogenation reactor (R2)together with a part of the saturated C₈ products (8) and fresh hydrogen(7). The remaining part of the saturated C₈ products and fresh hydrogen(11) is sent, on the other hand, to a second hydrogenation reactor (R3)together with fresh hydrogen (6) and the olefinic stream rich in heavyhydrocarbons (5). The stream (13) which is obtained at the outlet of thereactor forms the plant product.

When, on the other hand, it is the hydrogenated blend which isfractionated, the process according to the invention can comprise thefollowing steps:

-   -   a) dimerizing the isobutene contained in a C₄ cut (FCC, Coking,        Steam-Cracking, Dehydrogenation of isobutane);    -   b) sending the product leaving the dimerization reactor to a        first distillation column from whose head the C₄ products are        recovered, whereas the C₈-C₁₆ olefinic blend is recovered from        the bottom;    -   c) hydrogenating the C₈-C₁₆ olefinic blend with suitable        catalysts using a saturated hydrocarbon stream to dilute the        olefinic charge;    -   d) sending the hydrogenation product to one or more distillation        columns where the excess hydrogen is recovered, together with a        saturated stream rich in C₈ olefins, which is recycled to the        hydrogenation reactor, and a high-octane hydrocarbon blend        (which can also contain C₁₂ olefins).

The saturated stream rich in C₈ olefins recycled to the reactor, can besubstantially free of hydrocarbon compounds higher than C₈.

The saturated stream rich in C₈ olefins, which is recycled to thehydrogenation reactor, is in a weight ratio preferably ranging from 0.1to 10 with respect to the olefinic stream at the inlet of thehydrogenation reactor.

A simplified process scheme is shown in FIG. 3 to illustrate this newconfiguration more clearly.

The C₄ stream (1) containing isobutene is sent to the reactor (R1) inwhich the isobutene is selectively converted to dimers. The effluent (2)from the reactor is sent to a separation column (C1) where a stream (3)essentially containing the non-converted isobutene, linear olefins andsaturated C₄ products (n-butane and isobutane) is removed at the head,whereas a stream (4) consisting of dimers and higher oligomers isremoved at the bottom.

The bottom stream (4) is sent to the hydrogenation reactor (R2) togetherwith the stream of recycled product (9) and fresh hydrogen (5). Theeffluent from the reactor (7) is then sent to a second distillationcolumn (C2) from which the non-converted hydrogen (10) is recovered fromthe top, the product containing heavy C₁₂ and C₁₆ hydrocarbons (8) fromthe bottom and as side cut, a pure C₈ stream (9) which is recycled tothe reactor R2.

Optionally, for the separation of the effluent of the hydrogenationreactor, a solution which envisages the use of two distillation columns,can be used.

In both configurations, the hydrogenation catalysts adopted arepreferably based on nickel or noble metals.

Some examples are provided for a better illustration of the invention,but which should in no way be considered as limiting its scope.

EXAMPLE 1

This example illustrates a possible process application of the presentinvention. A hydrocarbon fraction, obtained by the selectivedimerization of isobutene and having the following composition:

C₈ olefins 90.0% by weight  C₁₂ olefins 9.5% by weight C₁₆ olefins 0.5%by weightis sent to a hydrogenation reactor (adiabatic with intermediate cooling)together with a stream consisting of saturated C₈ hydrocarbons (in aratio of 1:1) and a stream of hydrogen.

Using a commercial catalyst based on supported palladium and operatingin liquid phase with a space velocity of 1 h⁻¹ (volumes of olefin withrespect to the volume of catalyst per hour), a hydrogen pressure of 3MPa and an initial temperature of 140° C., the following conversions canbe obtained, per passage:

Conv. C₈ olefins 99.9% Conv. C₁₂ olefins 93.0% Conv. C₁₆ olefins 60.0%Conv. total olefins 99.1%

The reaction effluent is then sent to a distillation column from whosehead the excess hydrogen is recovered, as side cut, a saturated C₈stream (C₁₂<0.5% by weight), whereas the reaction product is recoveredat the bottom. Operating under these conditions, it is possible toobtain a hydrogenated product with a content of residual olefins lowerthan 1% by weight.

EXAMPLE 2

This examples illustrates another possible use of the process of thepresent invention which comprises the fractionation of the olefinicstream. A hydrocarbon fraction, obtained by the selective dimerizationof isobutene and having the following composition:

C₈ olefins 90.0% by weight  C₁₂ olefins 9.5% by weight C₁₆ olefins 0.5%by weightis sent to a fractionation column where the following two fractions areseparated:

Head (86%) C₈ olefins 99.5% C₁₂ olefins 0.5% Bottom (14%) C₈ olefins28.6% C₁₂ olefins 67.9% C₁₆ olefins 3.5%

The C₈ olefins collected at the head (86% of the total olefins) are sentto a first hydrogenation reactor (adiabatic with intermediate cooling)together with a stream consisting of saturated C₈ products (in a ratioof 1:1) and a stream of hydrogen.

Using a commercial catalyst based on supported palladium and operatingin liquid phase with a space velocity of 2 h⁻¹, a hydrogen pressure of 3MPa and an initial temperature of 130° C., 95% of the C₈ olefins areconverted, per passage.

The bottom product of the column is joined to the remaining part ofhydrogenated C₈ products (equal in mass to the olefins removed at thehead of the column so as to have a final stream still with a total of90% of C₈ hydrocarbons) and sent to a second hydrogenation reactorwhere, using a commercial catalyst based on supported palladium andoperating in liquid phase with a space velocity of 1 h⁻¹, a hydrogenpressure of 3 MPa and a temperature of 140° C., the followingconversions can be obtained, per passage:

Conv. C₈ olefins 99.9% Conv. C₁₂ olefins 93.0% Conv. C₁₆ olefins 60.0%Conv. total olefins 95.5%

Operating under these conditions, it is possible to obtain ahydrogenated product with a content of residual olefins lower than 1% byweight.

EXAMPLE 3 (COMPARATIVE)

This example shows how, using a classical hydrogenation scheme, it isnecessary to resort to much more drastic reaction conditions tocompletely eliminate the olefins from the product. In this case, infact, in order to control the reaction heat, a part of the product isrecycled to the reactor and consequently the content of residual olefinsmust be minimized.

The hydrogenation of the olefinic blend, whose composition is the sameas Examples 1 and 2, is always carried out in liquid phase with acommercial catalyst based on supported palladium, a hydrogen pressure of3 MPa but with a space velocity of 0.5 h⁻¹, and a temperature of 150°C., necessary for obtaining conversions of C₁₂ and C₁₆ olefins of over99%.

In this case, the process is much less economical with respect to theprevious examples (greater quantity of catalyst and highertemperatures).

1. A process for the production of hydrocarbon blends with a high octanenumber by the hydrogenation of hydrocarbon blends, containing branchedC₈, C₁₂ and C₁₆ olefinic cuts, characterized by sending said blends, assuch or fractionated into two streams, one substantially consisting ofthe branched C₈ olefinic cut, the other substantially containing thebranched C₁₂ and C₁₆ olefinic cuts, to a single hydrogenation zone or totwo hydrogenation zones in parallel, respectively, only the streamsubstantially consisting of saturated C₈ hydrocarbons, obtained by thefractionation of the stream produced by the single hydrogenation zone orobtained by the hydrogenation zone fed by the fractionated streamsubstantially consisting of the branched C₈ olefinic cut, being at leastpartly recycled to the single hydrogenation zone or to the hydrogenationzone fed by the fractionated stream substantially consisting of thebranched C₈ olefinic cut, and to said hydrogenation zone fed by thefractionated stream substantially containing the branched C₁₂ and C₁₆olefinic cuts.
 2. The process according to claim 1, wherein the branchedC₈, C₁₂ and C₁₆ olefinic cuts are oligomers of isobutene.
 3. The processaccording to claim 2, wherein the branched C₈, C₁₂ and C₁₆ olefiniccuts, oligomers of isobutene, derive from the dimerization of isobutene.4. The process according to claim 1, wherein the hydrocarbon blendsconsisting of branched C₈, C₁₂ and C₁₆ olefinic cuts also containbranched C₉-C₁₁ and C₁₃-C₁₅ olefinic cuts, in a smaller quantity.
 5. Theprocess according to claim 1, wherein part of the stream substantiallyconsisting of saturated C₈ hydrocarbons, obtained from the hydrogenationzone fed by the fractionated stream substantially consisting of thebranched C₈ olefinic cut, is sent to the hydrogenation zone fed by thefractionated stream substantially containing the branched C₁₂ and C₁₆olefinic cuts.
 6. The process according to claims 1, 2 or 3, comprisingthe following steps: a) dimerizing the isobutene contained in a C₄ cut;b) sending the product leaving the dimerization reactor to a firstdistillation column from whose head the C₄ products are recovered,together with, as side cut, a stream rich in branched C₈ olefins and asbottom product a stream rich in branched C₁₂ and C₁₆ olefins; c)hydrogenating, in a first reactor, the stream rich in branched C₈olefins, obtained as side cut, with suitable catalysts using a part ofthe same C₈ products already saturated to dilute the olefinic charge; d)hydrogenating with suitable catalysts, in a second reactor, the streamrich in branched C₁₂ and C₁₆ olefins together with the remaining part ofthe already saturated C₈ products, obtaining a saturated high-octanehydrocarbon blend.
 7. The process according to claim 1, wherein thestream rich in branched C₈ olefins removed as side cut is substantiallyfree of hydrocarbon compounds higher than C₈.
 8. The process accordingto claims 1 or 3, comprising the following steps: a) dimerizing theisobutene contained in a C₄ cut; b) sending the product leaving thedimerization reactor to a first distillation column from whose head theC₄ products are recovered, whereas the C₈-C₁₆ olefinic blend isrecovered from the bottom; c) hydrogenating the C₈-C₁₆ olefinic blendwith suitable catalysts using a saturated hydrocarbon stream to dilutethe olefinic charge; d) sending the hydrogenation product to one or moredistillation columns where the excess hydrogen is recovered, togetherwith a saturated stream rich in C₈ olefins, which is recycled to thehydrogenation reactor, and a high-octane hydrocarbon blend.
 9. Theprocess according to claim 1, wherein the saturated stream rich in C₈products recycled to the hydrogenation reactor, is in a weight ratioranging from 0.1 to 10 with respect to the olefinic stream at the inletof the hydrogenation reactor.
 10. The process according to claim 1,wherein the saturated stream rich in C₈ products recycled to thereactor, is substantially free of hydrocarbon compounds higher than C₈.11. The process according to claim 6, wherein the hydrogenationcatalysts are based on nickel or noble metals.
 12. The process accordingto claim 1, wherein the blends substantially consist of branched C₈-C₁₆olefins, wherein the branched C₁₂ olefins range from 3 to 20% by weight,the branched C₁₆ olefins range from 0.5 to 5% by weight, the remainingpercentage being the branched C₈ olefins.
 13. The process according toclaim 1, wherein the stream substantially consisting of saturated C₈hydrocarbons has a content of C₁₂ olefins less than or equal to 0.5% byweight.
 14. The process according to claim 1, wherein the streamsubstantially consisting of saturated C₈ hydrocarbons has a content ofsaturate C₈ greater than or equal to 99.5% by weight.